Peach trees generally produce a large amount of fruit, but crowded conditions create small peaches. Thin the fruit out within the first 40 days after bloom before the pit hardens. Leave about 6 to 8 inches between the remaining fruit.
Proper fertilization improves peach tree growth and vigor, affects current fruit yield and influences the growth of fruitwood for next year's yield.
Irrigation and direct sunlight also play important roles in fruit size.https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-thinning.htm